Thermal decomposition of gaseous silicon precursor compounds for the production of silicon nanowires is known. Besides various silicon compounds, catalytically active metals are employed. Generally speaking, catalyst metal agglomerates of a few nanometers in diameter are produced first, and then act catalytically on the decomposition of the silicon compounds and contribute to the ordered deposition of the elemental silicon formed. Depending on the reaction conditions, the resulting nanowires are crystalline or wholly or partly amorphous. It is preferred to use metals which exhibit eutectic mixtures with a low melting temperature with silicon. The model conception says that, under the reaction conditions, a liquid metal/Si mixture is formed, from which, finally, solid Si deposits as a result of further uptake of Si from the precursor compounds as they decompose. However, a comparable growth behavior is also observed even at temperatures below the eutectic melting point. Silicon nanowires are deposited predominantly on substrates such as silicon or such as metal oxides, an example being Al2O3.
For example, E. C. Garnett, W. Liang, and P. Yang, Advanced Materials 2007,79,2946, describe the production of silicon nanowires by CVD deposition from SiCl4/H2 with Pt as a catalyst metal under atmospheric pressure and at 805° C. Y. Zhang, Q. Zhang, N. Wang, Y. Yan, H. Zhou, and J. Zhu, Journal of Crystal Growth 226 (2001), pp 185-191, use a similar method under atmospheric pressure with an optimized temperature of 900° C., with Ni as a catalyst metal.
It is known that for the epitaxial deposition of silicon nanowires on crystalline silicon it is necessary first to remove the oxide layer of the substrate. Where chlorosilanes are used as precursor compounds, there is formation, together with hydrogen additionally present, of HCl, which reacts with the oxide layer (S. Ge, K. Jiang, X. Lu, Y. Chen, R. Wang, and S. Fan, Advanced Materials 2005, 17, 56). When chlorine-free precursor silanes are used, the same effect can be achieved by admixing HCl (S. Sharma, T. T. Kamins, and R. S. Williams, Journal of Crystal Growth 2004, 261, 613). For example, WO 2001/136412, after the production of suitable catalyst metal agglomerates, claims the successive use of at least two different precursor gas mixtures, of which the first mixture comprises either a chlorine-containing silane or, in addition to a silane, another chlorine source, and which ensures the start of growth, but requires comparatively high temperatures for the decomposition. Thereafter, the reaction temperature is lowered and a second precursor gas is used which has a lower decomposition temperature. Suitable precursor compounds cited are SiH4, Si2H6, SiCl4, and SiH2Cl2. Examples of suitable catalyst metals are Au, Al, Pt, Fe, Ti, Ga, Ni, Sn, or In. In addition to the conventional CVD technique for producing silicon nanowires, there are also references to Plasma Enhanced Sputter Deposition and Plasma Enhanced CVD, which allow a reduction in the reaction temperature.
W. I. Park, G. Zhenq, X-Jiang, B. Tian, and C. M. Lieber, Nano Letters 2008, 8, 3004, describe how at 400° C. and 10 torr pressure, the growth rate of silicon nanowires with Au as catalyst is 130 times greater for disilane, Si2H6, than for SiH4. Even with reaction temperatures optimized for SiH4, the growth rate lags behind that starting from disilane by a factor of 31. S. Akhtar, A. Tanaka, K. Usami, Y. Tsuchiya, and S. Oda, Thin Solid Films 2008, 517, 317, show that nanowires can be produced from Si2H6/H2 using Au catalyst even at a temperature of 350° C. and under a pressure of 3 torr. For example, JP 2006117475 A and JP 2007055840 A describe the production of Si nanowires at temperatures as low as 250-300° C., using disilane and trisilane as silicon sources, employing the metals Au, Ag, Fe, and/or Ni as catalysts, and setting a pressure during the reaction of 1-5 torr.
H.-Y. Tuan, D. C. Lee, T. Hanrath, and B. A. Korgel, Nano Letters 2005, 5, 681, show that formation of Si nanowires takes place even without a substrate in supercritical organic solvents at 400-520° C. and a pressure of 14.3-23.4 MPa. The catalyst metal used is Ni and, in addition to trisilane, Si3H8, the precursor compounds employed include octylsilane and phenylsilane. A. T. Heitsch, D. D. Fanfair, H.-Y. Tuan, and B. A. Korgel, Journal of the American Chemical Society 2008, 130, 5436, show that for trisilane as a precursor molecule, this reaction leads to Si nanowires even under atmospheric pressure and at boiling temperature (420-430° C.) with high-boiling organic solvents.
A disadvantage of the use of silanes (SinH2n+2) is their pyrophoric properties (self-ignitability in air), which hinder handling.
It could therefore be helpful to provide innovative nanowires by a new method from suitable precursors of the specified kind that are new for this purpose, as well as to provide a method for producing such nanowires.